This invention relates to optical mixing for heterodyne detection, and in particular to a form of mixing that is insensitive to differences between the state of polarisation (SOP) of the received transmitted (TX) signal and that of the local oscillator (LO) signal with which the (TX) signal is being heterodyned.
A form of such detection is known from the paper by L. D. Tzeng et al entitled "Polarization--insensitive coherent receiver using a double balanced optical hybrid system", TU 12, Proc. OFC '88 Tuesday Afternoon/32. The Tzeng et al optical mixer employs a single polarisation insensitive 3 db fibre coupler, two polarisation resolving beam splitters and two polarisation controllers to provide four optical outputs for detection by two balanced pairs of detectors.